Hydrocarbon resources such as petroleum or natural gas has come to be produced by excavation through wells (oil wells or gas wells, also collectively called “wells”) having a porous and permeable subterranean formation. As energy consumption increases, deeper wells are being drilled, reaching depths greater than 9000 m worldwide and greater than 6000 m in Japan. In wells that are continuously excavated, the productive zone is stimulated in order to continuously excavate hydrocarbon resources efficiently from subterranean formations of which permeability has decreased over time or subterranean formations of which permeability has gradually become insufficient. Acid treatment and fracturing are known as stimulation methods (Patent Document 1). Acid treatment is a method in which the permeability of the productive zone is increased by injecting a mixture of strong acids such as hydrochloric acid and hydrogen fluoride into the productive zone and dissolving the reaction components of bedrock (carbonates, clay minerals, silicates, and the like). However, various problems that accompany the use of strong acids have been identified, and increased costs, including various countermeasures, have also been pointed out. Thus, a method for forming a fracture in the productive zone using fluid pressure (“fracturing” or “hydraulic fracturing”) has received attention.
Hydraulic fracturing is a method in which a fracture is generated in the productive zone by fluid pressure such as water pressure (also simply called “hydraulic pressure” hereinafter). Generally, a vertical borehole is drilled, and then, the vertical borehole is curved and a horizontal borehole is drilled in a subterranean formation of several thousand meters underground. Fracturing fluid is then fed into the well at high pressure, and a fracture is produced by the hydraulic pressure in the deep subterranean productive zone (layer that produces the hydrocarbon resource such as petroleum or natural gas), and the productive zone is thereby stimulated to extract the hydrocarbon resource through the fracture. The efficacy of hydraulic fracturing has also been expected for the development of unconventional resources such as shale oil (oil that matures in shale) and shale gas.
A fracture formed by fluid pressure such as water pressure immediately closes due to formation pressure when the hydraulic pressure is no longer applied. To prevent a fracture closure, a proppant is included in the fracturing fluid (that is, the well treatment fluid used in fracturing), which is fed into the well, thereby placing the proppant in the fracture. Inorganic or organic materials are used as proppants included in fracturing fluid, but silica, alumina, and other inorganic particles have been conventionally used, and sand particles such as 20/40-mesh have been widely used because they are capable of preventing fracture closure in a high-depth subterranean environment under high temperature and high pressure over as long a period as possible.
To excavate (produce) a hydrocarbon resource from a productive zone, it is necessary that the gap which hydrocarbon resources can pass is formed by the proppant which are arranged to prevent closure of the fracture, as a result, permeability is secured. Permeability of gaps by the proppant is required in order to greatly affect the production efficiency of excavation of the hydrocarbon resource from the productive zone.
Various types of water-based, oil-based, and emulsion-based fluid are used as well treatment fluids such as fracturing fluid. Because the well treatment fluid must have the function of transporting the proppant to the location where the fracture is generated in the well, it generally must have a prescribed viscosity, good proppant dispersibility, ease of after-treatment, and low environmental load. Therefore, in addition to the proppant, various additives are used in the well treatment fluid, such as gelling agents, antiscale agents, acids for dissolving rock and the like, friction-reducing agents, and the like. For example, a composition comprising approximately from 90 to 95 mass % of water, approximately from 5 to 9 mass % of 20/40-mesh sand (proppant) and approximately from 0.5 to 1 mass % of additives may be used as the fluid composition for performing fracturing.
The idea of blending a degradable material into well treatment fluids is known from the perspectives of ease of after-treatment of the well treatment fluid and reduction of the environmental load thereof. For example, Patent Document 2 discloses that solid particles comprising degradable materials are included in a fracturing fluid, and examples of the shape of the solid particles comprising the degradable materials include spheres, rods, sheets, ribbons, fibers, and the like. Examples of fibers also include resin fibers, together with glass, ceramics, carbon, metals, and alloys.
Patent Documents 3 and 4 disclose slurry-like well treatment fluids containing degradable fiber and a proppant in a viscous carrier fluid, and well treatment methods for injecting these slurries. Patent Documents 3 and 4 also describe, as degradable fibers, fibers comprising polymers or copolymers of lactic acid or glycolic acid, or copolymers of lactic acid or glycolic acid with other hydroxy or carboxylic acid or hydroxycarboxylic acid containing components, and specifically, degradable fibers having fineness of 0.1 to 20 denier and fiber length of 2 to 25 mm.
The degradable fibers disclosed in Patent Documents 3 and 4 can prevent settling of the proppant in a viscous carrier fluid while the well treatment fluid containing the proppant is being transported to the location where well stimulation such as fracturing is performed, and furthermore, it is degraded and removed after well stimulation such as fracturing has been performed. However, as the distance for which the well treatment fluid is transported increases (that is, as the productive zone becomes deeper), the settlement inhibiting effect on the proppant contained in the well treatment fluid must be further improved.
Based on increased demand for securement of energy resources and environmental protection, particularly as excavation of unconventional resources expands, excavation conditions are becoming increasingly harsh. Thus, as a degradable fiber for use in well treatment fluid contained in well treatment fluids such as fracturing fluid, cementing fluid, temporary plug fluid, and completion fluid, there has come to be a demand for a degradable fiber that has an excellent settlement inhibiting effect on the proppant in the well treatment fluid and that also improves the well stimulation effect of fracturing or the like due to being hydrolyzable or biodegradable.